The skin is the largest organ in the body, responsible for 20% of its total weight and, together with the hair, nails and glands, it forms the integumentary system. The main function is protection, because skin can prevent environmental agents from causing harm, conserve fluids, assist with thermoregulation, participate in the production of vitamin D (required for the absorption of calcium and phosphate by the small intestine) and relay important information to the brain via touch and pressure. Three distinct layers of the skin exist: the epidermis, the deeper dermis and the hypodermis or subcutaneous layer of connective tissue (Figure 51.1).